Conventionally, a computer system providing a large-scale data storage service to a host computer exists. This system is known to comprise a storage apparatus to which the host computer is connected and a management apparatus of the storage apparatus.
The storage apparatus manages multiple hard disks by the RAID (Redundant Array of Independent/Inexpensive Disks) method. Then, [the storage apparatus] logicalizes physical storage areas of a large number of hard disks and provides the logicalized resources to the host computer as logical volumes. The host computer accesses the logical volumes to request the reading and writing of data.
One type of this kind of logicalization technology is known as Thin Provisioning. In this logicalization technology, the storage apparatus sets volumes in which the storage capacity is virtualized for the host computer. In accordance with the host computer's write access to the virtual volumes, the storage apparatus sequentially assigns real storage areas to the virtual volumes.
Therefore, this technology is effective compared with the method of assigning a large amount of storage areas in the logical volumes from the beginning in that the storage resources can be utilized efficiently. This technology is described in Japanese Patent Application ANo. 2003-015915A and Japanese Patent Application No. 2006-338341A.
The method for providing storage areas to virtual volumes is achieved by assembling multiple logical resources to which real storage areas are assigned respectively into volumes called pools and assigning the pools to the virtual volumes.
If the host computer makes a write access to the virtual volumes, the storage apparatus assigns a certain size of storage areas from the logical resources in the pool to the virtual volumes. By this method, the storage apparatus can save write data from the host computer.
Japanese Patent Application No. 2007-66259A discloses that, for the purpose of preventing the storage capacity distributed to the logical volumes from the magnetic disks which are inappropriate compared with the use of the virtual volumes as a result of automatic expansion of the virtual volume capacity by Thin Provisioning, that is, for ensuring that storage areas are not assigned from low-speed, low-quality magnetic disks to the virtual volumes enabling high-speed access for the host computer, when the storage system automatically expands the virtual volume capacity, storage areas are added to the volumes from the magnetic disks matching the characteristics of the volumes.